Both hypothyroid and hyperthyroid patients have been reported to be associated with an increased incidence of atherothrombotic events. We completed analysis of thyroid function tests, serum lipid parameters (with special respect to lipoprotein[a]) and various hemostatic parameters in previously thyroidectomized patients who underwent thyroid scan for follow-up of their thyroid cancer. At that time, the patients change their thyroid status from hyperthyroid (due to thyroid hormone suppression therapy) to temporarily hypothyroid (due to discontinuation of thyroid hormone suppression therapy.) The blood specimens obtained during these phases represent primarily systemic thyroid hormone effects as opposed to underlying diseases with confounding effects of inflammation, etc., in primary hypothyroidism and hyperthyroidism. Since all previous studies compared different groups of hypothyroid and hyperthyroid patients, the results obtained in our studies with the same patients in different thyroid status are expected to provide a clearer understanding of the effect of thyroid hormones on components of atherothrombosis.